The invention relates to a method for using usable data in data formats which cannot be directly processed, a geodetic device, a geodetic system, a computer program product, a computer data signal and a reference or data directory.
In many geodetic applications and systems, there is the necessity of frequent or continuous transmission of data between various devices. Predominantly transmitted data are those which have a time or space reference and may contain parameters of the measurement, measured values or general communication, such as, for example, error messages. Examples of such data are the current time, the location of a measuring device as well as any existing reference points, and direction, distance and angle relative to measuring points. The transmission may be effected by a large number of suitable wire-linked or wireless communication means, such as, for example, via a direct cable connection and directional or nondirectional radio data transmission.
Without restriction of the general usability of the method according to the invention and of the devices according to the invention, the application for a differential global navigation satellite system (D-GNSS), such as, for example, the global positioning system (GPS), is intended to serve here as an explanatory example. Data types and data formats which are described purely by way of example but can be used with a similar form in many further applications also relate thereto.
In differential GNSS, the position determination of a mobile unit, the so-called rover, is effected by data reception and data measurement relative to satellites as well as data reception of data measurements from at least one reference station. Since the position of the reference station is known and it likewise receives the identical signals of the satellites, some inaccuracies and errors can be eliminated by this differential correction method. By means of this method, a higher accuracy is possible than will be possible with a rover without a reference station. Such a station continuously transmits data from the received satellite signals to the rover. Depending on design, this may be raw data or already processed data.
In practice, however, reference stations are generally not installed newly for each measuring process but a procedure is effected which is based on an entire network of fixed reference stations which can also be used simultaneously by different users. In addition to the satellite-related data and the time, these stations also transmit specific information about themselves, such as, for example, their own reference station number. In addition, it is necessary to transmit technical data, such as, for example, antenna parameters, or correction parameters. Examples of such a correction parameter are atmospheric or geometric corrections which were determined, for example, in a network of permanent reference stations and associated network evaluation software and can now be used for correcting rover measurements as a function of the distance to the participating reference stations.
Since the utilization of the data of a reference station for various rovers should be possible, the transmission of the signals is nondirectional. However, this gives rise to the requirement that the communication also has to be possible with the various systems in the transmission range of a reference station, so that a form of standardization of transmitted data formats is necessary.
A standard for manufacturer-independent data formats which is used for such a transmission of data is specified by the Radio Technical Commission For Maritime Services (RTCM) in Alexandria, Va., USA. The term data format describes a complete information unit for transmission between geodetic devices. The information exchange takes place by means of identical or different data formats at identical or different data frequencies. The term data field describes a complete, defined data content having a specified value range. Data formats are composed, for example, of an initial identity code, different data fields and a final identity code with a possible checksum for data testing on receiving. The term data type describes the possible distinctness, such as, for example, length, without a sign or with a sign, of an information unit. Data types are used for describing data fields.
The standard RTCM V2.x applicable to date consists of one or more headers with data fields as basic information, to which additions can optionally be attached.
The occurrence of such additions in a data format is indicated by so-called flags as indicators. This means that each message transmitted has, after its introductory part, a sequence which indicates to the receiver whether and optionally which further additions follow. From this information, the recipient knows how he has to interpret and to process the data stream.
However, this solution of the prior art has the disadvantages that the number of possible permutations with the number of flags used is relatively small but nevertheless not every manufacturer has taken into account all possible permutations and the processing of the data associated therewith in his devices.
A solution known in the prior art for the limited number of flags consists in defining data formats, for example as selected permutations, and providing them with a continuous coding. From this number transmitted at the beginning of the communication, a device is able to derive the data format as a sequence of different data fields comprising data types and hence to evaluate the data fields.